1. Field of the Invention
The present invention relates to an image processing apparatus and method for compressing a multi-image composed of a plurality of images superimposed on the same screen to transmit it via a circuit network.
2. Related Background Art
Conventionally, image processing apparatuses have been known which compress a multi-image composed of a plurality of images superimposed on the same screen to transmit it via a circuit network. Conventional image processing apparatuses were configured as shown in a block diagram as shown in FIG. 13.
In FIG. 13, 101 is a first image signal input device for inputting the image, comprised of, for example, a video camera, 102 is a first voice signal input device for inputting the voice, comprised of, for example, a microphone. 103 is a second image signal input device, comprised of, for example, a still video camera. 104 is a second voice signal input device, comprised of a microphone, like the voice signal input device 102.
105 is an image signal switching unit, comprised of switches, for switching, based on a control signal from a system control unit 608, which of two image signals input from the first image signal input device 101 and the second image signal input device 103 is made a child screen.
106 is an image signal processing unit, comprised of a field memory and a synchronizing signal separation circuit, for implementing a so-called picture-in-picture function of superimposing one of two input image signals as a child screen on the same screen. 106 will be described later using FIG. 3.
607 is an image compression unit, comprised of a discrete cosine conversion circuit, a between-frame comparison circuit, and a quantizing circuit, for reducing the data amount of image signal by removing a redundant signal component contained in an image signal output from the image processing unit 606.
608 is a system control unit for controlling the entire image processing apparatus, comprised of a CPU, a ROM and a RAM. 109 is a voice addition unit, comprised of a resistor and an amplification circuit, for adding or synthesizing voice signals input respectively from the first voice signal input device 102 and the second voice signal input device 104. 110 is a voice signal processing unit, comprised of an A/D converter and a memory, for quantizing a voice signal added by the voice addition unit 109.
111 is a signal multiplexing unit, comprised of a data latch circuit and a parallel/serial conversion circuit, for multiplexing a compressed and quantized image signal output from the image compression unit 607, and a quantized voice signal output from the voice processing unit 110 to generate packet data for the data communication.
112 is a circuit network control unit (line network control unit), comprised of a relay circuit and a transformer circuit, for making network connection or disconnection with or from other terminal equipments via the circuit network. 113 is a line terminal unit for connecting this image processing apparatus with the circuit network using a modular connector. 114 is an operation panel unit comprised of switches for operating the image processing apparatus of this conventional example.
Next, the overall operation of the image processing apparatus configured in this way will be described.
First, the image signals input respectively from the first image signal input device 101 and the second image signal input device 103 are input via the image signal switching unit 105 into the image signal processing unit 606.
Then, upon receiving a control signal from the system control unit 108, when a switch of the image signal switching unit 105 is at the first terminal a side, a multi-image is formed in the image signal processing unit 606 with an image signal from the first image signal input device 101 as a parent screen, which multi-image is then output to the image compression unit 607. Also, when it is at the second terminal b side, a multi-image is formed with an image signal from the second image signal input device 103 as a parent screen, which multi-image is then output to the image compression unit 607.
Next, a multi-image signal composed of two image signals output from the image signal processing unit 606 superimposed is compressed by subjecting image difference between frames to discrete cosine conversion in the image compression unit 607, then quantized after the image compression, and output to the signal multiplexing unit 111 as discrete digital image data.
On the other hand, the voice signals input from the first voice signal input device 102 and the second voice signal input device 104 are added and synthesized in the voice addition unit 109. And the added and synthesized voice signal is supplied to the voice signal processing unit 110 to be formed into discrete digital voice data, and output to the signal multiplexing unit 111.
The signal multiplexing unit 111 incorporates these digital image data and digital voice data into packet data for use in the data communication, and then outputs it to the circuit network control unit 112. The circuit network control unit 112 transmits image data and voice data to a remote terminal equipment via the circuit network. In this way, the image data and voice data are transmitted to the remote terminal equipment.
However, in the conventional image processing apparatus, because a multi-image superimposed in the image signal processing unit 106 is compressed, the following inconveniences have been often encountered.
That is, for example, if one of two image signals is an animated image signal, and the other image signal is a still image signal in which a high resolution image such as a document is photographed, there is a problem that the character written on the document becomes obscure due to quantization error in image compression, if the compression ratio of image signal is raised.
Also, in the conventional apparatus, because the switching between an in-frame compression mode and a between-frame compression mode occurred in the image compression at the preset N-th frame, the between-frame image compression would occur, even when the correlation of image between the previous frame and the current frame was weak, for example, when the child screen was moved halfway during image compression of multi-image signal, resulting in a problem that the image quality was degraded or the image compressed data amount increased rapidly.